Currently, in communication systems, signals sent on legacy Very High Speed Digital Subscriber Line 1 (“VDSL2”) lines interfere with crosstalk estimation being performed by communication lines adhering to the emerging G.vector amendment of VDSL2. For example, within the emerging G.vector amendment to the VDSL2 standard, crosstalk coefficients between communication lines will be estimated using pilot signals sent during sync symbol periods.
In the legacy VDSL2 standard, sync symbols occur periodically after every 256 data symbols. On a particular legacy communication line, all tones transmit one of two complex points of a 4-QAM constellation during the sync symbol period. For example, the two complex points may be 00 corresponding to “1+j” or 11 corresponding to “−1−j”. For explanatory purposes only, the two complex points will be represented by “1” and “−1.” Normally, the value transmitted during the sync symbol period is the same from sync symbol to sync symbol. For example, a central office may transmit downstream the value “1” repeatedly on all active tones. When the central office receives an online reconfiguration request (OLR) from a customer-side equipment, the central office may then switch the value to “−1” and send “−1” repeatedly to the customer-side equipment. Such a transition is interpreted by the customer-side equipment as an acknowledgment of the OLR. Such an acknowledgment is referred to hereinafter as a sync-flag. A sequence of “1” values and “−1” values that is used to convey sync-flags (or the absence of a sync flag) will be referred to hereinafter as a flag sequence.
In the emerging G.vector amendment to the VDSL2 standard, the current consensus is that pilot signals will be sent on sync symbols. For example, mutual orthogonal pilot sequences may be sent instead of flag sequences downstream on sync symbols during the sync symbol periods to estimate crosstalk on communication lines. For instance, a pilot sequence may be assigned to each communication line and the pilot sequence may be sent downstream to the customer-side equipment. At the customer-side equipment, error samples are determined and fed back to the central office. At the central office, the error samples are correlated with the pilot sequences in order to obtain estimates for all of the crosstalk coefficients. The process of obtaining error samples may be repeated as needed to obtain a more accurate crosstalk estimate. These estimates will then be used to cancel crosstalk using precoding. Each symbol of the pilot sequence may be similar in nature to the legacy sync symbol. For instance, each symbol of the pilot sequence may also include the values “1” or “−1.”
When legacy VDSL2 communication lines (hereinafter “legacy communication lines”) and communication lines adhering to the G.vector amendment (hereinafter “G.vector communication lines”) are both present in a communication system, the flag sequences on the legacy communication lines may interfere with the crosstalk estimation being performed by the G.vector communication lines. For example, if a communication system has four G.vector communication lines, the pilot sequences may be (1,1,1,1), (1,1,−1,−1),(1,−1,1,−1) and (1,−1,−1, 1). Each pilot sequence is assigned to each of the 4 G.vector communication lines, respectively, and transmitted repeatedly on the sync symbols during the sync periods. Then to estimate the crosstalk from the second communication line into the third communication line, a sequence of four error samples measured at the customer-side equipment of the third communication line would be correlated with the pilot sequence of (1,1,−1,−1) assigned to the second communication line. Crosstalk from the other G.vector communication lines would not affect this measurement because of the orthogonality of the pilot sequences.
If the same communication system includes a legacy communication line, the central office would send the flag sequence by sending “1” repeatedly on all active tones until the central office received an OLR from the customer-side equipment, at which point the central office would acknowledge receipt of the OLR by sending a sync-flag by starting to send “−1” repeatedly on all active tones. If the flag sequence sent on the legacy communication line happened to be (1,1,−1,−1), coinciding with the pilot sequence sent on the second G.vector communication line, then the crosstalk estimate from the second G.vector communication line into the third G.vector communication line would be corrupted by crosstalk from the legacy communication line. As a result, the flag sequence on the legacy line could appear random, or could have consistent deterministic patterns, and thus controlling or predicting the level of contamination may be difficult.
The best existing solution is to shift the timing of the sync symbols of the legacy communication lines relative to the sync symbols of the G.vector communication lines. For example, the pilot sequences of the G.vector communication may be sent at one-time during a common sync period. Any legacy communication line may be shifted so that their sync symbols, containing flag sequences, occur in between common sync periods, while data symbols transmitted on the legacy communication line occur during the common sync periods. Although the sync symbols of the legacy communication lines are transmitted at a different time from the common sync period, interference with the crosstalk estimation may still occur because data symbols of the legacy communication lines will interfere with the crosstalk estimation.
For example, suppose that crosstalk estimation from a G.vector communication line i into a G.vector communication line k is estimated using a bipolar sequence of length L to modulate the sync symbols, and a legacy communication line is not present within the communication system. Focusing on a particular sub-carrier, the variance of the crosstalk coefficient estimate would be Nk/L, where Nk is the background noise power on G.vector communication line k. However, if a (mis-aligned) legacy communication line, line n, is present, then the variance of the estimate becomes Nk/L+|Hkn|2Pn/L, where Hkn is the relative crosstalk coefficient from the legacy communication line n into G.vector communication line k, and Pn is the transmitted power on the legacy communication line n. The additional variance in the crosstalk estimate indicates that crosstalk cancellation will be less accurate, or that it will take longer to achieve the same level of crosstalk cancellation. Furthermore, the additional variance term proportional to the power of the crosstalk from the legacy communication line into the G.vector communication line may be relatively significant.